A tread pattern formed on the tread surface of a tire known to the art generally comprises, as shown in FIG. 3, a design composed of main grooves 1, subgrooves 2, blocks 4 each having a thin kerf 3, etc. This tread pattern generally comprises a combination of pattern elements P each being a minimum unit wherein the same design repeatedly appears along the circumferential direction of the tire.
The pattern element has a plurality of types different from each other in the length in the circumferential direction of the tire. It is possible to reduce noises generated from the tire through provision of a proper combination of a plurality of types of pattern elements having different lengths over the entire portion in the circumferential direction of the tire. For example, increasing the ratio (.alpha.) of the length of the longest pattern element to that of the shortest pattern element among a plurality of types of pattern elements as large as possible based on the frequency modulation theory is said to be effective in preventing the frequency of noises generated from the tire from concentrating on a particular one.
However, when the difference in the length between pattern elements in the circumferential direction of the tire becomes large due to an increase in the ratio .alpha., the difference in the rigidity between the pattern elements is increased, which causes the element portions having smaller rigidity to be abraded at a higher rate than the rate of abrasion of the element portions having larger rigidity, i.e., brings about abnormal abrasion. Therefore, when the ratio .alpha. is increased, it is a common practice to reduce the difference in the length between adjacent pattern elements having different length through an increase in the number of types of pattern elements, thus reducing the difference in the rigidity. For example, in general, when the ratio .alpha. is around 1.3, the use of three types of pattern elements is sufficient for preventing the abnormal abrasion, while when the ratio .alpha. is increased to around 1.5 for the purpose of further reducing the noises generated from the tire, the occurrence of the abnormal abrasion cannot be prevented unless the number of types of the pattern elements is increased to about 5.
The length of the pattern element of the conventional tire is about 1% at the smallest relative to the length of the outer periphery of the tire. No tire comprising a combination of pattern elements having a length smaller than that of the above-described pattern element is known.
This is because a decrease in the length of the pattern element brings about a lowering in the rigidity of the pattern element which unfavorably results in the occurrence of abnormal abrasion and a lowering in the driving stability. Since the number of the pattern elements per peripheral length of the tire is about 80 at the largest although it depends upon the types of the tire, the average length of the pattern elements in the circumferential direction corresponds to about 1.25% of the length of the outer periphery of the tire. Therefore, under these conditions, even when the ratio (.alpha.) is increased, the maximum ratio of the ratio (.alpha.) is 1.5, because of the length of the shortest pattern element is about 1.0% relative to the outer periphery of the tire.
Further, an increase in the number of types of the pattern element makes the mold of the tire more complicated. This increases the cost for producing the mold, so that the production cost of the tire is increased.